1. Field of the Invention
This invention relates generally to a fuel cell system including a thermal sub-system that controls the temperature of a fuel cell stack and, more particularly, to a fuel cell system including a thermal sub-system that employs an on-line self-tuning control algorithm that controls the temperature of a fuel cell stack in response to system disturbances during operation.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
Various components in the fuel cell stack, such as the membranes, may be damaged if the temperature of the stack increases above a certain materials transition temperature, such as 85° C. A fuel cell system typically includes a thermal sub-system for maintaining the fuel cell stack at a desired operating temperature. The thermal sub-system typically includes a pump that pumps a cooling fluid through a coolant loop outside of the stack and cooling fluid flow channels provided within the bipolar plates. A radiator typically cools the hot cooling fluid that exits the stack before it is sent back to the stack.
It is known that precise relative humidity (RH) control is required for fuel cell stack performance and durability. One of the primary control parameters for RH is temperature. In order to accurately control the cooling fluid temperature, and thereby stack temperature, many factors need to be considered. A few of the known variables are radiator size and performance, cooling fluid type and stack size. However, there are many parameters that are unknown that should be treated as disturbances, such as ambient temperature, vehicle speed, stack power request and actuator dynamics. The problem with a fixed-gain controller is that they can only be tuned for one operating condition. When the conditions vary as much as they do in an automotive environment, a fixed-gain controller is typically inadequate.